1. Field of the Invention
The present invention relates to a mass spectrometer, particularly to a mass spectrometer used for MS/MS spectrometry or MSn spectrometry.
2. Description of the Related Art
A mass spectrometer includes an ionizing portion for ionizing a sample, and a mass spectrometric portion for subjecting a sample ion to mass separation and detecting the sample ion in accordance with a mass number ([mass]/[charge number]) and the like. At the ionizing portion, the sample is ionized by an electrospray ionizing method (ESI method), an atmospheric pressure chemical ionizing method (APCI method), a matrix-assisted laser desorption ionizing (MALDI) method or the like and delivered to the mass spectrometric portion. At the mass spectrometric portion, the sample ion is separated in accordance with the mass number by a separating mechanism of a sector type, a time of flight type (TOF) or the like and respective ions are detected by a detector of a microchannel plate or the like.
Here, in considering a case of analyzing a biopolymer of protein or the like, a normal mass measurement (MS measurement) for measuring the mass number of the sample ion is effective particularly for identifying the molecule. However, in the case of analyzing a molecular structure or carrying out database search having a high accuracy, an MS/MS measurement for fragmenting (fragmentation) a specific sample ion (precursor ion) and subjecting a fragment (fragment ion) to mass separation to measure is further effective.
Fragmentation of a sample ion is carried out by making the sample ion collide with a gas (collision gas) at a colliding portion.
As the colliding portion, a collision cell, an ion trap or the like is used (refer to U.S. Pat. No. 4,234,791 and JP-A-2002-184349). In the case of a TOF type mass spectrometer, there is also a case of providing a colliding portion at a partial region in a flight tube (refer to U.S. Pat. No. 5,202,563).
In the case of a collision cell, an ion and a gas are made to collide with each other during a time period of passing a sample ion through the cell to thereby subject the ion to cleavage. In the case of an ion trap, an ion falling in a range of a specific mass number gathered to a center of the ion trap by an electric field formed within the ion trap and gas are made to be collided with each other to thereby subject the ion to cleavage. Further, in the case of a flight tube, an ion and a gas are made to collide with each other during a time period in which a sample ion passes a colliding portion to thereby subject the ion to cleavage.
As a collision gas introduced to a colliding portion, it is necessary to select a gas having a pertinent mass in accordance with an ion (precursor ion) constituting an object and an ion (fragment ion) with an object of being intended to generate by cleavage. When a number of samples are intended to analyze continuously, or when a detailed structure analysis is intended to carry out by subjecting the sample to various cleavage even in the case of one kind of a sample, it is necessary to introduce a collision gas in accordance with the object into a cell. In a related-art apparatus, there poses a problem that a swift continuous analysis cannot be carried out since time is taken in switching such a collision gas.
Further, in the case of an ion trap, when an ion is caught and subjected to cleavage, a cooling gas is introduced into an ion catching space of the ion trap at a stage of selecting and catching the ion, and a collision gas is introduced at a stage of subjecting the ion to cleavage.
However, according to the related-art apparatus, only one kind of a gas is made to be able to be introduced into the ion catching space and a common gas is used for the cooling gas and the collision gas.
The cooling gas is operated to stabilize the ion in catching and selecting the ion and in order to carry out a highly accurate mass spectrometry, in cooling, it is necessary to prevent that the sample ion collides with the gas to be subjected to cleavage. Therefore, as the cooling gas, it is preferable to use a gas having a light mass. On the other hand, the collision gas is operated to subject the ion to cleavage by colliding with the sample ion. Therefore, as the collision gas, it is preferable to use a gas having a large mass.
However, according to the gas used in the related art, the mass is excessively large for cooling, the mass is excessively small for subjecting the ion to cleavage and therefore, mass spectrometry having a sufficient accuracy cannot be carried out by the related-art apparatus. Further, the ion cannot be subjected to cleavage efficiently.